Many packaging items such as bags and containers are destined, after a relatively short functional life, to arrive as a significant component of urban garbage or, to a much lesser extent, in the form of litter. Increasingly, such packaging items are being manufactured from plastics, that is to say from synthetic polymer compositions, which, whilst being cheap and having the physical properties which make them highly suitable for such packaging purposes, have the disadvantage, as components of waste or garbage, of being highly resistant to destruction by the chemical and physical actions to which they are subject in the natural environment and thereby such components constitute a mounting nuisance.
It is generally accepted that over 50% of the annual tonnage of all manufactured synthetic polymers are applied as packaging materials and that some 90% of this flow finishes as a component of urban garbage.
It has already been proposed that synthetic polymer compositions intended for packaging and the like purposes should be capable of being broken down chemically and physically by environmental action to which items made from such compositions become subject as litter. For example, it has been proposed that such compositions should be capable of being broken down biologically, i.e. should be `biodegradable`.
It has hitherto been acceptable to use the term `biodegradable` to describe polymer materials which might be environmentally acceptable when used as `one-trip` packaging materials but it is now clear that the chemical and physical processes at work in the interaction between the environment and these materials are more complex than was originally believed. Biodegradability can be defined as the degradation at the molecular level of substances by the action of enzymes themselves derived from the metabolic processes of micro-organisms. Such very specific interactions as have long been familiar in the case of natural polymers, for example that between cellulose and wood-decaying fungi, can be cited. In the field of synthetic polymers one can also find examples such as the destruction of certain polyester type polyurethanes by fungi, in particular Ulocladium chartarum.
Restricting consideration to that group of synthetic polymer types which are in general use in the packaging industry we find that these polymers are unaffected by enzymes derived from micro-organisms. A careful examination of circumstances in which such polymers appear occasionally to have been degraded by biological action has disclosed a high probability in each case that the process of degradation has proceeded in at least two stages, the first being chemical and resulting in a reduction of the molecular weight of some part of the polymer to a point at which the second, biological, stage can commence as an interaction between the micro-organisms of the environment and the low molecular weight material that has appeared. It is also recognised that the first, chemical, stage of this degradation process can follow different mechanisms which can be broadly classified under two headings the first being photochemical oxidative chain scission, and the second chemical catalytic oxidative chain scission.